<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            background-color: #ff0;

            /* 内容宽高 */
            width: 100px;
            height: 100px;

            /* 内边距 */
            padding: 50px;

            /* 边框 */
            border: 40px solid #000;

            /* 外间距 */
            margin: 30px;
        }
    </style>
</head>

<body>
    <div class="box">我是一个盒子</div>
</body>

</html>